Properties of Cobalt and Nickel in Aqueous Solution
Cobalt and nickel have very similar chemical properties. In aqueous solution both elements are normally found as divalent cations (e.g. Co++ or Ni++) in acid solution, with similar solubilities.
Both are soluble as the respective sulphates, chlorides, or nitrates in acid solution, but are largely insoluble in alkaline conditions unless chelating agents are present. (Both cations are chelated strongly by ammonia, for example). Their sulphide compounds have similar properties, (the KSP values are similar leading to a similar pH of formation with S2− ions), as do their respective carbonates.
This phenomenon presents an ongoing problem for the extractive metallurgist, as Co and Ni are invariably found together in naturally occurring ores, but must ultimately be separated to make maximum use of each metal. Fortunately however, there are exceptions to this pattern of similar behavior, which can be exploited, and will be discussed below.
Ratio of Cobalt to Nickel Occurring Naturally in Nickel Ores
Ni and Co commonly occur together in nature as sulphide ore deposits, and the ratio of Co:Ni in Ni ores is surprisingly constant in the range of about 1:15 up to 1:30. This is true at least in unaltered sulphide ores; although this ratio can be quite different if weathering of the ores has occurred, (over many millennia),
A general review of nickel metallurgy is to be found in the excellent book “The Winning of Nickel”, by Joseph P. Boldt Jr., and Paul Queneau Sr., published by Methuen and Co. 1967. A more recent review of laterites in particular is found in “The Past and Future of Nickel Laterites,” by Ashok Dalvi, Gordon Bacon, and Robert Osborne in: PDAC 2004 International Convention, Trade Show and Investors' Exchange, (Mar. 7-10, 2004).
Weathering of surface Ni deposits is common especially in tropical countries, and such deposits are usually referred to as Ni laterites; due to slight differences in chemistry, this weathering frequently results in a partial separation of Ni and Co over a vertical horizon, compared to the original sulphide ore. Some concentration of Co into the so-called limonite layer often is the result, so Co:Ni ratios in laterites considered for hydrometallurgical processing vary substantially from the usual ratio in sulphides, e.g. a ratio of 1:10 or even lower may be found in limonite.
Co Separation from Nickel in a Leach Solution . . . . Overview
The distribution of Co and Ni in most leaching processes is very similar. Thus leaching of Ni—Co ores or concentrates usually results in a mixed solution of Co and Ni, as well as other materials.
However, Ni and Co have to be separated eventually to obtain maximum use and payment for each element, as their end-use is significantly different, e.g. different metal alloys. With only a few notable exceptions, Ni is much more abundant in ores or concentrates than Co. Thus in a mixed Ni—Co solution, the problem of separation can be more accurately stated as separating Co (as an impurity, albeit of value) from a Ni solution. It also happens that Co has a few specific chemical properties that allow for its selective extraction from a mixture of elements in solution, whereas Ni in general does not have such properties.
A variety of processes have been (and some still are) used commercially to achieve this objective, but all of them have significant costs, and it is the objective of the present invention to provide a more efficient and more cost-effective method.
Also, because of the greater value of Co, this need to purify the Ni solution of Co should not obscure the secondary need to also recover Co itself in an economic manner, which is part of the present invention.
A number of processes have been used commercially for Co separation from Ni, for example:
Precipitation of Co from Ni—Co Solution as Co(OH)3 
This is an old process, (see the above-referenced book by Boldt and Queneau), one of the first known methods, and still used commercially. With strong oxidants, a Co++ solution can readily be oxidized to Co+++, which is essentially insoluble in dilute acid solution (say pH 2-6). Co is thus precipitated as Co(OH)3 whilst the Ni stays largely in solution. Oxidants used for this purpose include Cl2 and ozone. Electro-oxidation can also be used. However, the process is costly and inefficient, due to significant co-oxidation of Ni++ to a similar product, and has generally fallen out of favour.
Selective Solvent Extraction (SX) of Co from Ni—Co Solution
This approach has been the subject of many investigations, some of which have been commercialized, and some of these are listed in the referenced article, “Cobalt-Nickel Separation in Hydrometallurgy: a Review,” by Douglas S. Flett, in: Chemistry for Sustainable Development 12 (2004), pages 81-91. There are some organic extractants which will selectively extract Co with respect to Ni. Primarily these are one of two types:
i) Ternary and quaternary amines (Alamine™ 336 for example), which can extract some metal chloride complexes (e.g. CoCl42−) from a strong chloride aqueous environment. Unusually, Ni doesn't form such chloride complexes, so a good separation of Ni from other elements can sometimes be achieved. However, the requirement of the strong chloride concentration (several molar) severely limits the applicability of the process, and in reality omits it from serious consideration for a typical leach liquor.
ii) Phosphinic acids (e.g. bis 2,4,4-trimethylpentyl phosphinic acid, sold commercially as Cyanex™ 272), which will extract Co selectively over Ni, without the need for a chelating agent as in the amines. Although this extractant works well in pure solutions, unfortunately it also extracts many other metals commonly found in leach solutions, such as Mg and Mn, which limits its usefulness. This limitation is illustrated in the first two Examples described below. Although Mg can be scrubbed off the loaded organic stream by Ni/Co (with difficulty), by using a large number of mixer/settlers in counter-current mode, as was done at the Bulong plant (described by Donegan (“Direct Solvent Extraction of Nickel at Bulong Operations,” by S. Donegan, in: Minerals Engineering 19 (2006), pages 1234-1245), Mn cannot be scrubbed off. The only remedy is to co-extract all the Mn along with the Co, and pay for the cost of the Mn extraction/stripping, particularly the cost of the ammonia used for neutralization of the organic extractant. As a result of this limitation, Cyanex 272 is best applied after some prior purification, which itself is both costly and inefficient.
Selective SX of Ni from Ni—Co Solution
This approach was invented and commercialized at the Queensland Nickel (QNI) plant in Yabulu, Queensland, Australia, very successfully in the 80's, and is well summarized by the above-referenced Flett review article, and also by Reid and Price.
(Reid, J G and Price, M J, 1993. Ammoniacal solvent extraction at Queensland Nickel: Process installation and operation, in Solvent Extraction in the Process Industries Volume 1 (Proceedings of International Solvent Extraction Conference 1993) (eds: D H Logsdail and M J Slater), pp 225-231 (Elsevier Applied Science: London and New York.
The process is effective but suffers from high cost, as the major component (Ni), is being extracted away from the ‘impurity’ (Co), generally a more expensive route than the opposite. The impurities are all left with the Co.
Also the extractant used (hydroxyoxime) is prone to rapid degradation by Co II oxidation, and hence must be re-oximated on a regular basis, at considerable cost. Pre-oxidation of Co II to Co III is necessary to minimize this problem, but is not 100% effective, leading to continuous re-oximation of the extractant, at high cost.
Stripping of the Ni from the loaded organic can be done with either strong ammonia solutions (250 g/l NH3), or by acid, as was done at the Cawse mine in Western Australia for a while, as discussed by Flett (referenced above). The former fits in well with NiCO3 production, (by steam stripping of NH3), the latter with a Ni electrowinning flowsheet.
Although technically feasible, this approach is relatively expensive as noted, and does not produce a pure Co product, as the Ni left in the raffinate produces a Ni:Co ratio in this stream of at least 1:1. At Yabulu, a separate Co refinery had to be built eventually for re-processing of the Co-rich stream, and this refinery had its own technical and financial challenges.
Hydrogen Reduction of Ni from Ni—Co Solution
This approach was first commercialized in about 1950 at the Sherritt Gordon plant in Fort Saskatchewan, Alberta, as described by Boldt et al. The method has been considered the standard process for nickel recovery by some designers; it has been installed in several other nickel plants since, but suffers from significant drawbacks:                Batch mode operation. The process apparently can only be operated in this way, (instead of the usual continuous mode). This then requires multiple units (autoclaves), with low operating time (need to fill and discharge each batch autoclave frequently)        These features lead to high capital and operating costs        Heightened safety and occupational health requirements        Need for concentrated Ni solution (50-100 g/l [Ni]), and high solution feed temperatures, approx 200° C., again leading to high costs.        Need for careful control of pH, in pH 7.0 range, with high background levels of ammonium sulphate (200 g/l).        Large by-product production of ammonium sulphate crystals, at about 7× tonnage of Ni metal production, necessitating evaporative crystallizers, filters, dryers, bagging, storage facilities, etc. All leading to high costs.        Technically complex, thus requiring high level of technical skill, and expensive engineering input from a very limited number of qualified suppliers        Further processing costs, downstream from the actual hydrogen reduction due to need for further purification to remove trace amounts of impurities such as S and O. High temperature oxidation and reduction furnaces are needed for this purification.        Relatively poor quality of Ni product, regarding Co content        Poor quality of Co product, as the raffinate contains a Ni:Co ratio of about 1:1, leading to need for another Co refining step, similar to the process described in previous section.Difficulties with Separation of Cobalt from Nickel in the Presence of Impurities        
As described in the previous section, Co separation from Ni is difficult, and specific to each situation, e.g. solution chemistry and particularly the impurities in the Co—Ni solution. For solutions derived from leaching sulphide concentrates (as at Sherritt Gordon, for example), impurities are generally confined to other base metals such as Fe, Cu and Zn, which can be removed efficiently by known purification methods.
However, for acidic solutions derived from leaching of laterite ores, other impurities are found, particularly Mg and Mn, and often in much greater concentrations relative to the Ni and Co concentrations, e.g. as much as 10× greater. This situation makes it near difficult to use the preferred Cyanex 272 Co extraction method described above without some form of pre-treatment to separate Co and Ni from these impurities, or alternatively pay for the expense of co-extracting Mn.
Thus, unless Mn extraction is to be tolerated and paid for, treatment of laterite leach liquors is usually required to choose one of two routes:                Precipitation of Ni and Co in acid solution away from impurities as much as possible, followed by re-leaching to form a new solution with reduced impurity content. This approach for example was followed in the Murrin plant, which is described by Campbell et al in U.S. Pat. No. 7,387,767, and also in the Cawse plant, described by White in U.S. Pat. No. 6,409,979.        Leach in an ammoniacal alkaline environment, wherein most of damaging impurities are largely absent. This approach is adopted by the Caron process, for example (described in Boldt and Queneau), which was used at the QNI plant in Yabulu mentioned above, (now under different ownership and renamed).        
The Caron process requires a pretreatment process of its own, a reductive roast at high temperature. This is a pyrometallurgical process, and requires high capital investment. It also has high energy requirements and thus has high operating costs. For these reasons, it is generally not considered today, although a few plants built years ago are still operating.
Practically then, one is confined to the precipitation and releach option as a pre-treatment prior to Co—Ni separation.
For laterite ores therefore, it is desirable, even necessary, that the Mn/Mg be separated out from Ni/Co by first precipitating the Ni/Co from solution, and then releaching. This is usually done by one of two methods:                1. H2S precipitation of mixed Ni/Co sulphides, (which selectively precipitates Ni/Co over Mn/Mg), followed by filtration and then pressure oxidation of mixed sulphides precipitate, with associated filtration steps, to produce a Ni/Co solution suitable for efficient Cyanex 272 extraction of Co, (as is now done at Murrin Murrin, op cit), or        2. Mixed hydroxide precipitation of Ni/Co with MgO (which can be done selectively over Mn and Mg, as practiced at Cawse plant for example, and patented by White, op cit), or with CaO, which is not so selective with respect to Mg/Mn. At Cawse, this was followed by re-leaching of mixed hydroxides with ammonium carbonate solution, filtering, then steam stripping the NH3/CO2 from the leach liquor to precipitate Mn/Mg, (and re-adsorbing the same NH3/CO2), and re-filtering.        
Either of these processes for Mn/Mg rejection is expensive, especially in capital costs (H2S generating plant, or stripping and absorption plants for NH3/CO2). It is an objective of the present invention to be able to treat high Mn/Mg solutions containing both Ni and Co, and separate Co from this solution, without going through either of the existing Mn/Mg rejection alternatives sketched out above.
Unfortunately, a large amount of gypsum is formed along with the mixed Ni—Co hydroxide precipitate (MHP) when slaked lime is used as the precipitant for Ni and Co; consequently the MHP contains about 50 wt % gypsum, and only 50 wt % actual Ni and Co hydroxides, (and hence about 22% Ni).
Also both Mg and Mn are partly precipitated from solution (as hydroxides) at about the same pH as Ni and Co, thus further contaminating the product. This is a particular problem for laterites where these impurities are usually present in high concentration, e.g. in leach liquors derived from laterites by a High Pressure Acid Leach (HPAL) process.
Mn is of special interest as well because MHP is of course a mixture of Ni and Co compounds, which eventually have to be separated to make commercial Ni and Co products. The conventional technology for this separation is to use solvent extraction on a Ni/Co solution, in particular the extractant Cyanex 272. This Cyanex 272 separation is only feasible if Mn and Mg are very low in the feed solution, otherwise they interfere with the Co extraction. Generally this limits such refining to feed materials that are already low in Mn: Ni sulphide concentrates generally fall into this category, or are smelted to matte anyway, which is an effective Mn removal step, (into the slag). For feed materials that have not been smelted, e.g. laterite high pressure acid leach liquors, a significant Mn presence may be a serious impediment to further processing.
Commercial Value of Nickel Hydroxide Products
Mixed Ni—Co hydroxide (MHP) has been produced by at least one Ni mine in the recent past, and several announced projects have included this intermediate in the process flowsheet. However, this product has uncertain market value and a limited marketing history, due in part to its purity and grade. Since Ni and Co must be separated eventually, the presence of other impurities in the MHP can be a serious impediment to said separation, as described in the previous section.
If slaked lime, (Ca(OH)2), is used as the reagent, it typically precipitates some Mg and Mn from solution along with Ni and Co; and if any traces of base metal impurities, (e.g. Cu, Fe, Zn and Cd), are left in solution from the prior purification, they are also precipitated into the MHP. In addition, gypsum is formed of course, thus degrading the Ni grade of product by about 50%. If MgO is used instead of slaked lime, better selectivity is found in respect to Mg, and of course no gypsum is formed, so the Ni grade is much better, but most of the same impurity issues remain.
All of the above can affect on the marketing of the MHP, and hence its commercial value. Nevertheless for projects with limited Ni/Co production it may be advantageous to be able to sell the MHP as an intermediate product, and thus avoid the extra cost of full refining to metal products on a small scale. It was of some importance therefore to improve the quality of MHP, by Co separation upstream.
Therefore if the Co is separated out before the precipitation process, the resultant Ni Hydroxide Product (NHP) may have enhanced value.
U.S. Pat. No. 6,171,564 relates to a process for treatment of nickel ores and concentrates to recover both Ni and Co as refined metals. It is a ‘comprehensive’ process in the sense that:                a) Both sulphides and oxides (laterites) are considered as suitable feeds, and        b) The process describes a complete flowsheet going all the way to metal product:                    Acid leaching of solid feed to produce an acidic leach solution, containing Ni, Co and numerous impurities            purification of leach solution, in several steps, including solvent extraction            precipitation of Ni and Co together as an impure mixed hydroxide by neutralizing acidic solution            Releaching of mixed hydroxide in (recycled) ammoniacal solution to redissolve Ni and Co values            Separation of Co from Ni in said solution by solvent extraction, followed by stripping and recovery of Co product            Further purification of (ammoniacal) raffinate from Co solvent extraction by additional solvent extraction steps            Extraction of Ni from purified ammoniacal solution, followed by acidic stripping of organic stream to form purified acidic Ni solution            Ni recovery from purified solution by electrowinning, with recycling of spent acid to solvent extraction                        
The Ni and Co content in the feed material are first leached by pressure oxidation (in the case of sulphides) or by acid pressure leaching (in the case of laterites); then the solution is purified to remove primarily Cu, Zn and Fe. From the purified solution, Ni and Co are precipitated together at about pH 7-8, using slaked lime, as a mixture of Ni and Co hydroxides, (MHP).
MHP is then re-leached in mild conditions, (ambient temperature, dilute solids, neutral pH) with a strong ammonium sulphate solution (200 g/l) at about pH 7.0, as in Reaction (1):Ni(OH)2+(NH4)2SO4→Ni(NH3)2SO4+2H2O  (1)
Thus this leach produces a solution of Ni and Co diammine, (Ni(NH3)2++ and Co(NH3)2++ ions), which also contains some impurities, notably Mg and Ca.
The leaching of Ni and Co by this method is not very efficient, (about 90%), due to the mild conditions selected, i.e. the neutral pH and hence very low free ammonia content of the leach solution; almost none of the Ni species in solution is present as free ammonia, NH3. As a consequence, significant Ni and Co are left behind in the residue, which therefore has to be releached to avoid unacceptable losses. However, the choice of the neutral pH is very important to enable the subsequent solvent extraction steps to proceed efficiently.
Co is then extracted selectively from this impure Ni/Co diammine solution using as extractant the Cyanex 272 reagent at about this same pH 7.0, leaving Ni in the raffinate.
Since the extraction is from a diammine, (i.e. Co(NH3)2SO4), no neutralization is required to maintain a constant pH, as the ammine is simultaneously converted to ammonia (NH3) during the extraction, which exactly balances the acid production (H+ ions) from the organic reagent, and thus produces ammonium sulphate in the overall extraction reaction (2):Co(NH3)2SO4+2RH (organic reagent)→CoR2 (organic phase)+(NH4)2SO4  (2)
Thus the pH stays almost constant throughout the Co extraction, negating the usual need for neutralizing agent. This is an unusual and most beneficial feature of this solvent extraction process, as most other Co and Ni solvent extraction processes need in situ neutralization with ammonia or caustic to counteract acid production, and thus maintain the solution pH within the required range during the reaction, (or else the extraction stops prematurely). The significance of the neutralization goes far beyond the simple avoidance of reagent consumption; the normal byproduct of such neutralization with ammonia or caustic is a salt such as ammonium sulphate or sodium sulphate, which rapidly accumulates in the raffinate stream, and must be disposed of in some fashion. This is a serious challenge, given the constraints of the system, such as metal contamination of said salt as a potential byproduct, and is sometimes fatal to a process design.
Co extraction at this point is only about 90% of the Co contained in the diammine solution, and co-extraction of Ni and Mg is negligible, thus providing a relatively pure Co stream (in the loaded organic), in the absence of Mn, Fe, Cu or Zn (all of which can co-extract with Co). Co extraction is kept deliberately less than 100%, to ensure that the loaded organic (loaded organic) is fully loaded with Co, thus minimizing co-extraction of Mg and Ni, (which are less strongly extracted than Co). Even then, some scrubbing of loaded organic is required to remove the small amounts of Mg and Ni that are extracted. Scrub feed is derived from a fraction of the (pure) cobalt strip liquor, which is in limited supply, since the Ni:Co ratio in the solution is typically >10:1; so minimizing of scrubbing requirements is essential.
Co is then stripped from the loaded organic in conventional fashion with dilute sulphuric acid solution to form a concentrated and pure Co solution, (low in Mg and Ni), and then recovered from this strip solution by conventional electrowinning (as pure metallic cathode), or by precipitation with some suitable reagent (e.g. sodium carbonate) as a pure Co salt, carbonate or hydroxide, etc.
Cyanex 272 is applied as an extractant again at pH 7.0-7.5 to the Co raffinate, to recover the residual Co, (about 10% of feed Co in ammonium sulphate solution), and also to remove any Mg and Ca from this solution prior to Ni recovery. This is called the magnesium extraction stage for sake of reference. About 10% of feed Ni is also extracted here into the loaded organic, leaving about 90% of feed Ni in raffinate from this operation. Stripping of the loaded organic stream with acid produces an acidic aqueous stream which is recycled to the leach circuit for recovery of Ni/Co values. Co in raffinate is very low, (˜1 ppm [Co]), in order to produce high purity Ni in the next step.
Ni is extracted from Mg raffinate with LIX™ 84 extractant at about pH 7.0-7.5, as in Reaction (3);Ni(NH3)2SO4+2RH (organic reagent)→NiR2 (organic phase)+(NH4)2SO4  (3)
The Ni extraction is followed by acidic stripping of the loaded organic, to produce a pure Ni electrolyte, (4), and recovery of Ni metal as cathodes by conventional electrowinning, reaction (5):NiR2 (organic phase)+H2SO4→2RH (organic phase)+H2SO4+H2O  (4)NiSO4+H2O→Ni0↓+H2SO4+½O2  (5)
The final Ni raffinate is recycled to the original ammonium sulphate leach, completing the circuit.
It is to be noticed that the overall reaction, i.e. combining leaching of Ni hydroxide, solvent extraction, solvent stripping and electrowinning, reaction (6), has no reagent consumed and no byproducts, other than water and oxygen:Ni(OH)2→Ni0↓+H2O+½O2  (6)
Numerous other steps, (e.g. washing, scrubbing and stripping), are combined with each of the three main extractions, i.e. Co, Mg and Ni. Altogether about 40 individual mixer-settlers are used, making for quite a complicated and expensive process for Co removal, hence the incentive to make it simpler.
Finally it is worth emphasizing that the Co extraction by solvent extraction in U.S. Pat. No. 6,171,564 is only performed after first precipitating a mixed hydroxide, and then re-leaching this solid.
In case it might be wondered why this sequence is required, it is our contention that Co extraction by solvent extraction is quite inefficient if such precipitation/re-leach is not carried out, due to the impurities present in the feed liquor to precipitation.
In other words the Co solvent extraction with Cyanex 272 when applied to the (acidic) feed liquor to this precipitation is inefficient as it contains too many interfering impurities, particularly Mg and Mn; both of these impurities compete with Co in the acidic solutions.
Effectively this means that the hydroxide precipitate has to stay as a mixed hydroxide, containing both Ni and Co, as well as Mg and Mn, which limits its marketability in practice. It is worth noting that most Ni refineries now use Cyanex 272 for separating Co from Ni, so feed materials to these refineries are usually restricted to low Mg and low Mn materials. In practice, this means feed materials to Ni refineries are currently either Ni mattes or mixed sulphides, not mixed hydroxides or concentrates; hence the marketability for mixed hydroxide has not been established so far, and represents a significant uncertainty for any mine project that depends on selling such a product at a good price.
Reference is also made to patent applications WO 02/22896 A1; WO 02/22897 A1; WO 2005/073415 A1; WO 2005/073416 A1 and WO 2006/032097 A1 of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) which has been engaged for some years in researching the field known as Synergistic Solvent Extraction (SSX). This technique makes use of two or more extractants combined together to achieve beneficial effects superior to those of the individual extractants.
Co and/or Ni extraction from solution is the subject of a number of these patent applications, but generally they are extracted together from other impurities. The concept of trying to separate Ni from Co is evidently not contemplated in any of these patent applications.
Also, these patent applications do not contemplate trying to take advantage of kinetic differences between Ni and Co in extraction, which is an important and unusual feature of the present invention, i.e. unusual in solvent extraction.
Rather, extraction efficiencies in these prior applications by CSIRO are generally based on steady-state results, i.e. results which approach equilibrium, (the normal situation in solvent extraction).
Patent Application WO 2005/073415 discloses a process for selectively extracting Co and/or Mn from leach solutions containing Mg, Ca (and possibly Mn), using a combination of two organic extractants. This combination is made up of a carboxylic acid such as Versatic™ 10 (2-methyl, 2-ethyl heptanoic acid) and a hydroxyoxime such as LIX™ 63 (5,8-diethyl-7-hydroxy-6-dodecanone oxime).
The extraction process with this blend has superior selectivity possibilities for Co, Ni and Mn over the impurities mentioned, compared to extraction with Versatic 10 alone.
This improvement is expressed as a downward (synergistic) shift in the isotherm for Co, Ni and Mn (the pH50 is reduced by 1-3 pH units). Cu and Zn behave similarly (synergistic shift), whereas Mg and Ca behaved in the opposite sense, i.e. antagonistic shifts to higher pH50.
Taken together, this picture indicates improved separation possibilities for Co, Mn, Ni, Cu and Zn from Mg and Ca. However, it is noted that extraction of Ni with this organic combination is relatively slow compared to Co and Mn; Example 2 and FIG. 3 indicate that 10 minutes extraction is needed to get even 85% Ni extraction, (compared to <1 minute for Co and Zn), whereas the isotherm (where steady state conditions apply) indicates almost quantitative Ni extraction is possible at say pH 5. The implications are that the Ni extraction kinetics aren't fast enough to allow for a practical process for Ni extraction, and thus the focus is primarily on Co and/or Mn extraction.
Stripping of Mn and Co from the loaded organic (LO) is accomplished quickly with dilute acid, presumably at ambient temperatures (not specified though).
Considering the overall process, (extraction plus scrubbing and stripping), Co can be extracted together with Mn from a leach solution, and thus separated from Mg and Ca; alternatively Co can be extracted preferentially from Mn as well, presumably by operating at a lower pH. Small concentrations of Mn can also be scrubbed from the LO by Co if desired, (presumably this only works if Mn extraction is modest compared to Co loading).
As explained above, Ni extraction with this system is not attractive, as the Ni extraction kinetics are too slow to be useful, even though the Ni isotherm is similar to that of Co. Some Ni will extract inevitably though if present in the feed solution, and has to be stripped with Co or separated out subsequent to stripping. So Ni is more of a nuisance to a Co purification process, if anything. If present, Cu and Zn also extract in a similar fashion to Co, and must be separated out by selective stripping or in subsequent steps on the strip product stream.
Thus this process is particularly aimed at Co extraction away from common leach liquor impurities, particularly Mg and Ca, where the Co is the only metal of interest (i.e. Cu, Zn and Ni are missing or in minor concentrations), and is not particularly attractive for solutions that contain high Ni as well as Co.
Patent Application WO 2005/073416 is particularly aimed at Co and/or Ni extraction away from common leach liquor impurities, particularly Mn, Mg and Ca. It uses a similar organic mixture as WO 2005/073415, e.g. Versatic 10 and LIX 63, except that a 3rd component is added, a so-called kinetic accelerator like TBP.
This 3rd component literally accelerates the extraction of Ni, so that both Ni and Co are extracted together, and thus compensates for the perceived slow extraction kinetics of Ni. It also accelerates the stripping of Ni from the organic.
The benefits of the synergistic system together with the accelerator are described in Examples 1-8, and shown in FIGS. 4-9, 11 and 12.
The process has several embodiments, distinguished largely by the design of the stripping circuit, to separate Ni from Co, after the two metals have been co-extracted into the organic phase. Co strips more easily than Ni, so selective stripping is an option for separating the two metals, using mild conditions, e.g. with dilute acid or at higher pH.
Thus in Option 1 (page 13), Co is selectively stripped from the loaded organic, leaving the Ni behind for later stripping under more severe conditions. This option is shown as flowsheets in FIGS. 1-3, and described in Examples 9 and 10.
In Option 2 (page 16), Co and Ni are stripped together, and then the strip product liquor containing both metals (but notably free from Mg) is then subjected to a 2nd extraction, typically using Cyanex 272, which is selective for Co over Ni, (as noted above) in the absence of Mg and other metals. This option is described in Example 11, and shown in FIG. 10.
Various complications in both options arise with other metals, e.g. Cu, Mn and Zn, which co-extract with Co and Ni, and have to be subsequently separated out during stripping, or by scrubbing, from Co and Ni.
In none of the examples shown, nor in the text or claims, is Co separated from the Ni in the solution by selective extraction, as in the present invention.